I am trying to built a WiFi communication with Arduino using ESP8266 module. There is one servo, one step motor connected to Arduino and whenever the ESP8266 Wi-Fi module receives a signal, the voltage level on the motor and servo fluctuate dramatically, which causes inaccurate results on how motor and servo behaves. I have connected the ESP8266 module using SoftwareSerial, and even when the signal is not transferred into motor or sevo, the voltage levels change whenever a data is received by the WiFi module.

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This might be an issue with having a weak (or not stabilized) power supply. When system momentarily draws to much current it drops the voltage. What do you use to power your circuit?
– Filip FranikAug 7 '18 at 11:03

When it is connected to USB or a 7.5 V power supply, creates the same issue.
– Uğur KayaAug 7 '18 at 11:12